A salt marsh has three distinct zones: mud flat, low marsh, and high marsh. The seemingly barren mudflats, exposed only at low tide, teem with creatures like fiddler crabs, mud snails, and marine worms. These invertebrates provide food for the birds, mammals, and fish that live in the marsh. Growing between mid-tide and high-tide is salt marsh cord grass (Spartina alterniflora), the only plant that grows in the low marsh. Spartina has a mutualistic relationship with the fiddler crabs and ribbed mussels that live in the low marsh. Fiddler crab burrows aerate the roots of Spartina, providing essential oxygen to the plants, while the nutrient-rich waste created by mussels provides Spartina with nitrogen. In turn, the Spartina binds the soil, provides a surface to which mussels can attach, and provides shelter and food to the fiddler crabs. Through sediment trapping, and filtering, conversion, and uptake of nutrient, heavy metals and other toxins, the organisms in the low marsh naturally reduce pollution levels in our waters. The high marsh is immersed only in the highest tides of the month (called spring tides), and the plants growing there are less tolerant of salt than Spartina. Salt meadow cord grass (Spartina patens) is the dominant high marsh plant and can be easily identified by the way wind and water swirl its tender stems into ridges that resemble cowlicks. Other high marsh species include spike rush (Eleocharis parvula), salt-grass (Distichlis spicata) and black grass (Juncus gerardii).